1. Field of the Invention
The present invention relates to an antenna with which a digital signal or an analog high-frequency signal, e.g., that of a microwave range or an extremely high frequency range, is transmitted or received.
2. Description of the Related Art
For two reasons, wireless devices are desired which are capable of operating in a much wider band than conventionally. A first reason is the need for supporting short-range wireless communication systems, for which the authorities have given permission to use a wide frequency band. A second reason is the need for a single terminal device that is capable of supporting a plurality of communication systems which use different frequencies.
For example, a frequency band from 3.1 GHz to 10.6 GHz, which has been allocated by the authorities to short-range fast communication systems, corresponds to a bandwidth ratio as wide as 109.5%. As used herein, “a bandwidth ratio” is a bandwidth, normalized by the center frequency f0, of a band. On the other hand, patch antennas and ½ effective wavelength slot antennas, both of which are known as basic antenna structures, have operating bands (as converted to bandwidth ratios) of less than 5% and less than 10%, respectively, and thus cannot realize the above-described widebandness. To take for example the frequency bands which are currently used for wireless communications around the world, a bandwidth ratio of about 30% is required in order to cover from the 1.8 GHz band to the 2.4 GHz band with the same antenna. In order to simultaneously cover the 800 MHz band and the 2 GHz band, a bandwidth ratio of about 90% must be similarly realized. In order to simultaneously cover from the 800 MHz band to the 2.4 GHz band, a bandwidth ratio of 100% or more is required. Thus, as the number of systems to be supported by the same terminal device increases, and as the frequency band to be covered becomes wider, the need will increase for a small-sized wideband antenna.
The open ended ¼ effective wavelength slot antenna, shown in schematic diagrams in FIGS. 22A to 22C, is one of the most basic planar antenna structures (Conventional Example 1). FIG. 22A is an upper schematic see-through view; FIG. 22B is a schematic cross-sectional view taken along line AB; and FIG. 22C is a schematic see-through rear view, as seen through the upper face side. As is shown in FIGS. 22A to 22C, a feed line 113 exists on the upper face of a dielectric substrate 101. A recess is formed in a depth direction 109a from an outer edge 105a of an infinite ground conductor 103, which in itself is provided on the rear face of the dielectric substrate 101. Thus, it functions as a resonator composed of a slot 111 having an open leading end at an open point 107. The slot 111 is a circuit which is obtained by removing the conductor completely across the thickness direction in a partial region of the ground conductor 103, and which resonates near a frequency fs such that its slot length Ls corresponds to a ¼ effective wavelength. The feed line 113, which partially intersects the slot 111, electromagnetically excites the slot 111. The feed line 113 is connected to an external circuit via an input terminal. Note that, in order to establish input matching, a distance Lm from an open end point 119 of the feed line 113 to the slot 111 is typically set to about a ¼ effective wavelength at the frequency fs. Moreover, a line width W1 of the feed line 113 is typically designed so that the characteristic impedance of the feed line 113 is set to 50 Ω, in accordance with the substrate thickness H and a dielectric constant of the substrate.
As shown in FIG. 23, Japanese Laid-Open Patent Publication No. 2004-336328 (hereinafter “Patent Document 1”) discloses a structure for operating the ¼ effective wavelength slot antenna shown in Conventional Example 1 at a plurality of resonant frequencies (Conventional Example 2). Although the band can be expanded through operation at a plurality of resonant frequencies, characteristics which are as ultrawideband as currently desired cannot be obtained with the frequency characteristics shown in Patent Document 1.
Non-Patent Document 1 (“A Novel Broadband Microstrip-Fed Wide Slot Antenna With Double Rejection Zeros” IEEE Antennas and Wireless Propagation Letters, vol. 2, 2003, pages 194 to 196) discloses a method for realizing a wideband operation of a slot resonator with short-circuited both ends, which is a ½ effective wavelength slot antenna (Conventional Example 3). One input matching method for a conventional slot antenna has been to intersect and excite the slot resonator 14 at a point where a ¼ effective wavelength at the frequency fs is obtained, beginning from the open end point 119 of the feed line 113. However, in Conventional Example 3, as shown in FIG. 24 (which shows an upper schematic see-through view), the region spanning a distance Lind from the open end point 119 of the feed line 113 is replaced by a transmission line having a characteristic impedance higher than 50 Ω. The resultant inductive region 121 is coupled to the slot 111 in a substantial center thereof. Herein, Lind is set to a ¼ effective wavelength at the frequency f0, so that the inductive region 121 functions as a separate ¼ wavelength resonator from the slot resonator. This increases the number of resonators within the equivalent circuit structure (which is one in usual slot antennas) into two, and since resonators that are resonating at close frequencies are coupled to each other, a multiple resonance operation is obtained. In the example shown in FIG. 2(b) of Non-Patent Document 1, reflected impedance characteristics as good as −10 dB or less are obtained with a bandwidth ratio 32% (from near 4.1 GHz to near 5.7 GHz). As shown in comparison with respect to the measured characteristics in FIG. 4 of Non-Patent Document 1, the bandwidth ratio of the antenna of Conventional Example 3 are much more wideband than the bandwidth ratio of 9% of a usual slot antenna which is supposedly produced under the same substrate conditions.
Moreover, Non-Patent Document 2 (“Impedance Measurement of the Antenna on the Portable Telephone using Fiber-optics”, 2003 Grand Meeting of the Institute of Electronics, Information, and Communication Engineers, B-1-206 2003, page 206; Conventional Example 4) reports that, in a small-sized communication terminal in which the ground conductor area that can be secured for antenna operation is finite, use of an unbalanced feed circuit for feeding will allow an unbalanced ground conductor current occurring in the ground conductor to flow back to the ground conductor of the feed circuit, thus affecting the measurement accuracy of radiation characteristics and impedance characteristics itself. For this reason, Non-Patent Document 2 does not use a high-frequency unbalanced feed circuit for feeding. Rather, as shown in FIG. 25, Non-Patent Document 2 takes the trouble of employing optical fibers to ensure that the ground conductor in the communication terminal is fed in an isolated manner from the feeding system, thus adopting a measurement technique which avoids unfavorable influences of an unbalanced ground conductor current in the small-sized antenna.
As described above, conventional slot antennas not only lack sufficient widebandness but also have a problem in that, even if widebandness is realized within a small shape, their radiation characteristics and reflected impedance characteristics may not remain stable depending on the state of connection with an external unbalanced feed circuit, thus making it difficult to know their characteristics when mounted in terminal devices.
Firstly, as in Conventional Example 1, the operating band of a usual open ended slot antenna, which only has a single resonator structure within its structure, is restricted by the resonation mode band, so that the frequency band in which good reflected impedance characteristics can be obtained only amounts to a bandwidth ratio of less than about 10%.
Although Conventional Example 2 realizes a wideband operation because of a capacitive reactance element being introduced in the slot, it is well conceivable that an additional part such as a chip capacitor is required as the actual capacitive reactance element, and that variations in the characteristics of the newly-introduced additional part may cause the antenna characteristics to vary. Moreover, judging from the examples disclosed in FIG. 14 and FIG. 18 of this document, it is difficult to realize low-return input matching characteristics across an ultrawide band.
In Conventional Example 3, the bandwidth ratio characteristics are only as goods as about 35%. Moreover, use of a slot resonator with short-circuited both ends (which is a ½ effective wavelength resonator) is disadvantageous in terms of downsizing as compared to the antennas of Conventional Example 1 and Conventional Example 2, in which an open ended slot resonator (which is a ¼ effective wavelength resonator) is used.
Even if the principles of multiple resonance operation of Conventional Example 3 are introduced into the ¼ effective wavelength slot antenna design of Conventional Example 1 or Conventional Example 2, when the small-sized antenna operates as shown in Conventional Example 4, an unbalanced ground conductor current will flow back to the ground conductor of the unbalanced feed circuit which is connected to the antenna. Depending on the shape of the unbalanced feed circuit in which an unbalanced ground conductor flows, e.g., the length of a coaxial cable which is connected to the antenna for the purpose of knowing its characteristics, the radiation characteristics and reflected impedance characteristics will change. In particular, the radiation characteristics may drastically change depending on the state of the external circuit.